The present invention relates generally to photopolymerized printing plates useful in letterpress, and other similar printing operations. More specifically, the present invention relates to a shallow relief photopolymer printing plate and methods for producing and using such a plate.
Photopolymer printing plates have found widespread and successful use in letterpress printing processes, particularly in the newspaper industry. Conventional photopolymer plates hold many advantages over prior art, metal etched printing plates. For example, the time required to make the photopolymer plates is considerably shorter and, with the introduction of water developable photopolymers, problems relating to environmental contamination have been significantly reduced. In addition the photopolymer plates are much easier to handle than are the metal etched plates.
In spite of their widespread acceptance in the industry, photopolymer plates do suffer from the disadvantage of being relatively expensive, particularly when compared to the plates used in stereotype systems utilized by some of the large newspapers. Thus, there is a need in the industry for a less expensive photopolymer printing plate which will enable photopolymer printing systems to economically compete with existing stereotype systems.
Conventional photopolymer printing plates utilize photosensitive materials which are deposited on a supporting substrate such as metal or plastic. It has been found through experience that, in order to obtain acceptable printing quality, it is necessary that the photosensitive layer have a thickness greater than about 0.020 inch. When photopolymer plates having a photosensitive layer of less than about 0.020 inch are utilized in letterpress machines a "bottoming" (printing on white or background areas) occurs. Of course, such a bottoming effect is unacceptable and, therefore, the photopolymer printing plates heretofore used have required a relief thickness and therefore a photosensitive layer of at least 0.020 inch.
In accordance with the present invention, a photopolymer printing plate is provided having a photosensitive layer with a thickness substantially less than that of prior art printing plates. Thus, the cost of the printing plates manufactured in accord with the present invention is substantially less than that of conventional photopolymer printing plates. In addition, the printing plates of the present invention may be used on letterpress machines to produce printed material of high quality without bottoming in background areas.
Accordingly, the present invention is generally directed to a shallow relief printing plate comprising a photopolymerized layer which is supported on a substrate. The photopolymerized layer includes both raised image or "relief" areas and background areas, the background areas having an array of small protuberances thereon. In addition, the present invention is directed to photopolymerizable elements and methods for making and processing such elements to provide this shallow relief printing plate. Finally, the present invention is further directed to printing processes making use of the shallow relief printing plates disclosed herein.